Blends of cis-polybutadiene with either natural rubber or cis-polyisoprene, method of preparing same, and tire tread comprising same



. 3,669,939 BLENDS OF CliS-POLYBUTADENE WITH EITHER NATURAL RUBBER R CI-PQLYISPRENE, METHOD OF PREPARING SAME, AND TIRE TREAD COMPRISHNG SAME Henry E. Raiisbaek and Nelson A. Stumpe, In, Bartiesville, Okla, assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Aug. 17, 1959, Ser. No. 833,975 10 Claims. (Cl. 15233ti) This invention relates to blends of cis-polybutadiene with natural rubber and with cis-polyisoprene.

This is a continuation-impart of our copending application Serial No. 751,150, filed July 28, 1958, now abandoned, for Blends of Cis-Polybutadiene with Natural Rubber. Application Serial Number 751,150 was a continuation-impart of our application Serial Number 699,- 187 (now abandoned), filed on November 27, 1957.

Various methods are described in the literature for polymerizing 1,3-butadiene, including emulsion polymerization, alkali metal-catalyzed polymerization, and alfincatalyzed polymerization. Emulsion polymerization of 1,3'-butad iene gives a polymer with from about 60 to about 80 percent trans IA-addition, from about 5 to about 20 percent cis 1,4-addition, and from about 15 to about 20 percent 1,2-addition. Sodium-catalyzed polybutadiene has from about 60 to about 75 percent 1,2- addition, the remainder being cis and trans 1,4-addition. When potassum and other alkali metals are employed as catalyst, the latter ratios may vary to some degree, but no polybutadiene prepared in presence of the alkali catalyst containing more than about 35 percent of cis 1,4 configuration has been obtained. Alfin-catalyzed polybutadiene has from about 65 to about 75 percent trans 1,4-addition, from about 5 to about percent cis 1,4 addition, and from about 20 to about 25 percent 1,2- a'cldition. For a more complete discussion of the configuration of p'olybutadiene, reference is made to an article by J. L. Binder appearing in Industrial and Engineering Chemistry No. 46, 1927 (August 1954).

Natural rubber has long been a commodity of com merce, and the art is well aware of such natural rubbers. The most widely used rubber latex is that gathered from the tree Hevea brasiliensis. It is generally accepted today that natural rubber is substantially a cis-polymer of isoprene (2-methyl-1,3-butadiene). It is also known to the art that natural rubber fluctuates widely in price, but has generally been more expensive than the synthetic rubbers, especially since World War II when the production of various synthetic rubbers has increased many fold. Various synthetic rubbers and natural rubbers have been blended with each other and with fillers for various purposes. However, when synthetic rubber and natural rubber are blended, the properties of the blend generally trend toward the poorer of the two components. This is especially true in respect to heat build-up and flex life, and, consequently, the use of blends of synthetic rubber with natural rubber in utilities where natural is normal ly used alone has not proven satisfactory.

Although natural rubber has been utilized for a number of years in a multitude of applications, one of the largest uses at the present time is in the manufacture of truck tires. While certain synthetic rubbers, SBR

(styrene/butadiene copolyrner rubber) for example, show superior tread wear, natural rubber is superior in hysteresis properties. The poorer hysteresis of synthetic rubber, or blends of previously known syntheticfrubbers with natural rubber, results in premature failure on account of heat blowouts, particularly in heavy. duty tires. This problem is so acute that practically all large truck tires are made using natural rubber. The supply of natural rubber has been short in the past several years, and with the production of automobile and truck tires being increased every year, it is easily foreseen that the supply of natural rubber will become even shorter. It is known in the art thatbutadiene can be polymerized to polymers consisting primarily of cis'1,4-'co'nfiguration. However, while the polybutadienes containing sub stantial amounts of cis 1,4-configuration have excellent utility in many applications, the higher Mooney polybutadi'enes (ML-4 at 212 F. of and -above). have exhibitedpo orer processing characteristics than SBRor natural rubber, e.g., are more difiicult to mill, These cis-polybutadienes have excellent hysteresis properties, but it would be desirable if thetensile strength of these polymers could be increased significantly.

It has now been found that blends ofthe'se cis-polyb'utadi'enes with natural rubber and withcis-polyisjopren'e have good hysteresis properties, low heat build-up, good flex life,.and.good tensile strength; 1

. It is therefore an object of this invention to provide blends of cis-polybutadiene with natural rubber and with cis-polyisoprene which can be utilized in the applications for which natural rubber alone is used at the present time.

it is another object of thisinvention to provide blends of cis polybutadien'e with natural rubber andwith cispol yiso'pi'ene, which exhibit good hysteresis properties, good tensile strength, good flex life and low heat build-up,

It is another object of this invention to provide a means of extending natural rubber without sacrificing its desirable properties for tires, especially heavy dutytypes.

It is still another object of this invention to provide blends containing cis-polybutadiene, which have good processing properties. I H I p Still other objects, advantages and features of this invention will be apparent .to those skilled in the art upon consideration of this disclosure.

T he above and other objects of invention are accomplished by preparing a blend of rubber containing 10 to parts per 1 ()0 parts by, weight rubber of a synthetic polybutadiene containing at least percent butadiene joined together by cis 1,4 linkage, the remainder being chiefly a cisp olyisoprene, i.e,, natural rubber or a synthetic cis-polyisoprene containing at least 75 percent isoprene joined together by cis 1,4-linkage Of course, additives such as carbon black, antioxidants, softeners and other additives and preservatives known in the art can be present in the blend.

The cis-polybutadienes which are utilized in the rub: ber compositions of this invention can be produced by any of the known polymerization processes which yield predominantly cis-1,4butadierie' polymers. The cispolyb utadiene which can be employed in the nlbber com positions of this invention will have a viscosity between 10 and as measured on the Mooney viscosimeter at 212 F. (ML-4). A more desirable range of Mooney viscosity is from 20 to 60, inclusive. The polybutadiene as contemplated herein is one in which at least 75 percent and up to 100 percent, preferably 85 to 98 percent, of the polymer is formed by cis 1,4-addition of the butadione, the remainder of the polymer being formed by trans 1,4- and 1,2-addition of the butadiene. The amount of the cis-polybutadiene which is employed in the blends of this invention, the cis 1,4 content of the polymer, and the Mooney viscosity of the cis-polybutadiene will all depend upon the desired ultimate use of the blend and the physical properties desired for the ultimate use. In general, the blend will contain at least 10 weight percent natural rubber or cis-polyisoprene and preferably at least 25 weight percent. A particularly preferred range is 50 to 60 weight percent natural rubber or cis-polyisoprene and 50 to 40 weight percent of the cis-polybut-adiene.

As Has been indicated the cis-polybutadiene useful in this invention can be prepared by any method known to the art, this invention being in the blended composition. One means for preparing such polymers is fully described and claimed in the copending application of David R. Smith and Robert P. Zelinski filed April 16, 1956, and having Serial No. 578,166. According to that application, 1,3-butadiene is polymerized in the presence of a catalyst composition comprising (a) a trialk'ylaluminum, and (b) titanium tetraiodide. The polybutadiene produced by that method is one in which the rubbery polymer is formed by cis 1,4-additi-on, trans 1,4-addition and IQ-addition, at least 85 percent of the polymer being formed by cis 1,4-addition.

The trialkylaluminum in the Smith et al. catalyst can be represented by the formula R Al, wherein R is an alkyl radical containing up to and including 6 carbon atoms. The alkyl groups can be either straight or branched chain alkyl, for example, ethyl, propyl, isopropyl, n-butyl, isobutyl, pentyl, isoheXyl and n-heXyl etc. The alkyls can be-the same or different, e.g., diisobutylmonoethylaluminum; however, the preferred catalyst comprises titanium tetraiodide and triethylaluminum or triisobutylaluminum since these latter two alkylaluminums have high activity in the process. The amount of trialkylaluminum in the catalyst composition is usually in the range 1.25 to 50 mols per mol of titanium tetraiodide with the preferred range being from 1.5 to 35 mols per mol. When triisobutylaluminum is utilized, the preferred range is 1.7 to 35 mols per mol whereas when triethylaluminum is employed the preferred range is 1.5 to 10 mols per mol. The total amount of catalyst can vary over a wide range. The concentration of the total catalyst, titanium tetraiodide plus trialkylaluminum, is usually in the range of about 0.05 weight percent to 10.0 weight percent or higher, preferably in the range 0.05 to weight percent, based on the total amount of 1,3-butadiene charged to the reaction zone. In general, at the lower mol ratio of trialkylaluminum to titanium tetraiodide, it is desirable to operate above the minimum level of catalyst concentration.

The polymerization of the butadiene can be carried out at any temperature in the range of 40 C. to 150 C., but it is preferred to operate in the range of to 50 C. It is also preferred to carry out the polymerization in the presence of an inert hydrocarbon such as aromatics, straight and branched chain parafiins and cycloparaflins although cycloparaflins are less desirable than the other hydrocarbons. absence of any such diluent. The polymerization reaction can be carried out under autogenous pressure or any suitable pressure sufficient to maintain the reaction mixture substantially in the liquid phase. The pressure will thus depend upon the particular diluent being employed and The reaction can be carried out in the 7 essing aids.

the temperature at which the polymerization is being carried out. However, higher pressures can be employed if desired, these higher pressures being obtained by some such suitable method as the pressurization of the reactor with a gas which is inert with respect to the polymerization reaction.

As has been indicated, natural rubber is well known to the art and no further discussion thereof is needed here.

The cis-polyisoprenes used in the rubber composition of this invention can be produced by any of the known polymerization processes which yield a predominantly cis 1,4-polymer of isoprene. The cis-polyisoprene is one in which at least 75 percent and up to 100 percent, preferably to percent, of the polymer is formed by cis 1,4- addition of the isoprene, the remainder of the polymer being formed by trans 1,4-, 3,4-, and 1,2-addition of the isoprene. The amount of the polyisoprene formed by 1,2-addition is usually negligible, being in most instances diflicult or impossible to detect by infrared examination.

In one method for preparing cis-polyisoprene, the isoprene is polymerized in the presence of a catalyst composition comprising (a) a trialkylaluminum and (b) titanium tetrachloride. The trialkylaluminum can be repre sented by the formula R Al, wherein R is an alkyl radical as described hereinbefore. The polymerization is preferably carried out in the presence of a hydrocarbon diluent similar to that mentioned above. The amount of titanium tetrachloride used in the catalyst composition is usually in the range of 0.05 to 20 mols per mol of trialkylaluminum. However, a preferred range is from 0.1 to 3.0 mols of the titanium tetrachloride per mol of trialkylaluminum. The process for preparing the cis-polyisoprene can be carried out at any temperature within the range of C. to 100 0., but it is preferred to operate in the range of 50 C. to 50 C. The polymerization reaction can be carried out under autogenous pressures. It is usually desirable to operate at pressures suflicient to maintain the monomeric material substantially in the liquid phase. The amount of the catalyst composition used in the polymerization can vary over a wide range. The concentration of the total catalyst composition is usually in the range of about 0.01 weight percent to 15.0 weight percent, or higher, based on the amount of isoprene charged to the polymerization zone. A polyisoprene prepared by this method is formed by cis 1,4-addition, trans 1,4-addition, 3,4-addition and 1,2-addition, at least 90 percent of the polymer usually being formed by cis 1,4-addition.

It is to be understood that it is not intended to limit the invention to cis-polybutadienes or cis-polyisoprenes which have been prepared by any particular method. Thus, the present invention is applicable to any cis-polybutadienes and cis-polyisoprenes having the above-described configurations. Another method which can be used to produce cis-poiybutadienes suitable for use in preparing the blends of this invention is described in copending US. patent application Serial No. 722,842, filed on March 21, 1958, by F. E. Naylor, now Patent No. 3,004,018. As disclosed in this application in detail, a catalyst comprising a mercury or zinc-aikyl and titanium tetraiodide is effective in polymerizing 1,3-butadiene to a cis-polybutadiene. Furthermore, other catalyst systems, e.g., those containing elemental lithium or lithium hydrocarbon, such as alkyllithiums are suitable for use in preparing cis-polyisoprene by the polymerization of isoprene.

The blends of this invention can be prepared in a variety of ways, but the preferred method for admixing these cis-polybutadienes with natural rubber or cis-polyisoprene is with mechanical mixers such as roll mills or Banburys, either with or without plasticizers, peptizers or other proc- After admixing the natural rubber or cispolyisoprene with the cis-polybutadiene in the desired ratio, the resulting blend can be compounded and vulcanized by well known rubber vulcanization recipes, as for spectrometer. The percent of the total unsaturation present as trans 1,4- was calculated according to the following equation and consistent units r E example, sulfur plus an accelerator at 307 F. for 30 min- 1% utes. Alternatively, each polymer can be compounded 5 h t. m Hi t 1 1 i \V ere: 629x 1H0 OD. CO6 C1811 1 ers-mo S -IIl CI'ODS separately and the compounded stocks blended to give the Ezexfinction (g 10) t=Path length (micronsl; aDd desired ratio of cis-polybutadiene to natural rubber or c:ctz nt entratign Ii11Slg%l15b1e b0I1d{)1it i11)- 'r l i e exg ncgo was 6 ermine a e m-IQTOB an an e er; 1110 OH. CIS'POIYISOPRPHB the final blend- The r esultmg f coefiieient used was 1.21 10- (1iters-mols -microns- The after vulcanization, have excellent physical properties, percent ofl the total ll lsatlil'attifiln p'esent a 1, 2- (01'iYl11l11) was ca (311 a 8 3.000! I!" 0 e a 0V8 e 113-1 11, US 11 e and Show Rartlcular advaniages H} that low heat E 10 11.0 micron band 3.1161 3; extinction coeffi ieht of 1.52%10- good flex life, good abrasion resistance, and good resistl ip i-l)- pe e g 0f ta g 011 S811 {LSCS 0 3.118 S C111 6133.113 to agmg P 1 A st1n fflrther advanjxflge 1 ,4- grid 1,2-(v-i nyl) d e tirmined acgo'r ing to the above in the blends of this invention is their processability. i net l i ods 0m the htlbeoretitcal satu ration z iss i i n 's on e on per eac 1 uni in e p0 ymer. n e case 0 Whlle cls'polybutadlenes havmg P Y value of 50 Polymer A, the value given is an average value calculated or above (ML-4 at 212 F.) are dlfi'icult to mill, the from the structure of each of the polymers of the blend. blends of this invention mill well on a roll mill. Other Th b1 d blending methods include blending solutions of the two y gi i lg g 'g g gz i i 5: ggfi g ig r rubbers and recoverin the blend from solution. t, if. g after milling in the smoked sheet. The blends were then EXAMPLES 2O compounded according to the following recipes, followed by curing at 307 F. for 30 minutes. It was noticed that Several runs were made to illustrate the advantage of the 60 and 54 Mooney elrs-polyblltadlenes the blends of this invention. These runs are presented to (Polymers B and See Table d not band at the o illustrate the rubber compositions of this invention but m111 temperature p y after addlng are not intended to limit the invention to the embodiment natural the mlxtul'e banded y Satisfactoshown therein. rily at 158 F.

Example I COMPOUNDING RECIPES Several runs were made by blending cis-polybutadiene prepared with triisobutylalurninum and titanium tetra- N 1 iodide catalyst and natural rubber #1 Smoked Sheet gis polybutadiene gk (#1 SS). sheet The cis-polybutadiene was prepared by the method of Smith et al. in toluene diluent utilizing a mol ratio of tri- I II III .IV isobutylaluminum (TIBA) to titanium tetraiodide (TTI) I of about 5/1. Three different cis-polybutadienes were Y :33:: 13 9 28 used in these blends, these polymers having the following j b r furnace black g 2 5g 52 s 1110 0X1 8 C18 1 AT-Content. Steario acid 3 3 3 3 Antioxidant 1 1 1 1 1 Cis 1,4-add1t1on, percent 40 Disproportionated rosin 5 g 3 3 Polymer A 1 V 1.75 1.75 1.75 1.75 Polymer B 89.0 n-g g lg xyl-l enz t i uli n- 1 0 0 85 0 7 0 4 Polymer C 935 1 1 (1 th 1 i oximatel e um Physicalmixture containing 65% byweight of a complex diarylarni'neg 5 gfi gg i gg of f i g these pglyrgers 21L) 5 ketongreaction product and 35% by weight 01 N,N-diphenyl-p-phen. mol ratio of triisobutylaluminum to titanium tetraiodide was ylene lamme' utilized, whereas a 5/1 mol ratio was utilized in the preparation of the other two. After curing, the physical properties of the blends were In determining the percentage of the polymer formed by cis Let-addition, the following procedure was followed. The determlned- The results of these tests are glven 1n polymers were dissolved in carbon disulfide containing 0.01 Table I grain of phenyl-beta-naphthylamine per liter of carbon disul- 7 tidf to forilnf ahsolutlion containing 265 weitght plercetnt 0g thte From the table, it can be seen that the heat build-up, o yzner, t e o yiner as prepare con aine an ioxi an g 5 was removed 5 reprecipimtmg the polymer twice fyom tensile strength, flex llf e, etc., of the blend, especially the cy ge prior to p p g the carbon lfi e u 50/50 blends, is essentially as good as that of the smoked The infrared spectrum of each of the solutions (percent h transmission) was then determined in a commercial infrared 5 act alone- TABLE I Polymer A (81.7% 015- Polymer B (89.0% cis- Polymer 7Q (93.5% cis- Natural Mooney vis- 1,4); Mooney vis- Mooney visrubber cosity (ML-4), 18 cisoty (ML-4), eosity (ML-4), 54 alone (control) Run No 1 2 3 4 5 6 7 8 9 10 Parts by weight:

Ois-polybutadiene 100 50 75 50 100 75 50 0 Natural rubber 0 25 50 o 25 50 I 0 25 50 100 Processing Data Compounded Mooney (MS 1% at 212 F.) 24.5 28 32.5 68.5 v 62.5 47 60 58.5 49.5 48 Scorch at 280 F.,2 minutes to 5 point Mooney rise 14. 5 l3. 5 13. 5 10. 5 11. 5 12. 5 1O 11 11. 5 11. 5 l l l l f 250 a 45 3 50 1 45 5 20 3 23 1 35 6 33 4 3e 3 40 s 0 e5 min B' 51.5 Grams/min 94 101.5 85.0 37.5 68.0 87.0' 71.0 81.5 90.0 95.0 Appearance rarinr 11 11+ 1]. 6 7- 9- I 6' (H- v 9+ 11-' See footnotes at end of table.

TABLE IContinued Cured 30 Minutes at 307 F.

Polymer A (81.7X cis- Polymer 13 (89.0X cis- Polymer (93.5X cis- Natural 1,4) Mooney vis- 1,4) Mooney vis- 1,4) Mooney visrubber cosity (MM), 18 cosity (ML-4), 60 cosity (ML-4), 54 alone 7 (control) Run No 1 2 3 4 5 6 7 8 9 10 Compression set, percent 4 25. 5 22. 8 21.3 16. i 16. 7 17. 3 14.8 15.3 16.2 21.7 300% modulus, p.s.i., 80 FL- 990 1,000 1, 220 l 1, 400 1,480 1,440 1,450 1, 140 1,330 1, 430 Tensile strength, p.s.1., 80 F. 2, 240 2, 120 3, 000 f 1, 600 2, 660 l 3, 600 2, 200 2, 580 3, 390 1 3, 400 Elon%ation, percent, 80 FL. 530 550 525 l 320 440 f 550 400 470 540 f 505 200 Max. tenslle, p.s.i. 910 1, 480 740 1,070 1, 600 2,430 1,170 1, 640 2,180 2,750 Heat bu1ld-up, A '1, F 5 61. 5 58.1 55.1 50. 3 44. 2 44. 6 38.8 44. 6 43. 9 47. 9 Resilience, percent 7 66. 7 64. 0 66. 4 73. 3 70. 4 71. 8 77. 5 72.1 71. 2 67. 7 Flex life, thousands of flexures to failure 8 6. 2 11.0 22.0 2. 5 8. 4 20. 0 1. 2 3. 5 20. 9 d 13 Shore A hardness 68 06. 5 67 72 G9 67 70. 5 71 68. 5 65 Time to blowout, minutes 7. 7 8. 0 10. 3 18. 9 2 12.8 e 30 16. 7 8. 6 Gehruan freeze point, C." 91 75 59 84 68 -63 57 Ozone rating: 13

Three days 5 8 3 5 3 3 4 3 3 2 Seven days 10 10 10 10 10 10 10 10 10 4 ASTM D927-55T, Mooney viscometer, small rotor, 212 F., 1.5

minutes.

2 ASTM Dl077-55T, Mooney viscometer, large rotor, Scorch in minutes to 5 point rise above minimum Mooney.

8 No. l/2 Boyle extruder with Garvey die. See Ind. Eng. Chem. 34, 1309 (1942). As regards the rating" figure, l2 designates an extruded product considered to be perfectly formed whereas lower numerals indicate less perfect products.

4 ASTM D39555, Method B (modified). Compression devices are used with 0.325 inch Spacers to give a static compression for the one-half inch pellet of percent. Test run for 2 hours at 212 F. plus relaxation for 1 hour at 212 F.

.AS'IM D412-51l. Scott tensile machine L-G. Tests are made at 80 F. unless otherwise designated.

5 ASIM D623-52T. Method A, Goodrich flexometer, 143 lbs/sq. in. load, 0.175 inch stroke. Test specimen is a right circular cylinder 0.7

inch in diameter and 1 inch high.

7 ASTM D945-55 (modified). Yerzley oscillograph. To; specimen A is a right circular cylinder 0.7 inch in diameter and 1 inch 111 ASTM D8l3-52T (modified). DeMattia flexing machine. punctured specimen is subjected to a bending action at a constant rate under certain conditions of stroke and temperature and the rate of crack growth measured. The DeMattia tester is used in these tests with a 3-inch stroke, 3-inch wide test specimen with 3 pierces in groove, and 500 flexurcs per minute at 210 F. The results are reported as thousands of fiexures to complete break.

9 ASTM D676-55T. Shore durorneter, Type A.

Exam ple' II A number of runs were made wherein natural rubber was blended with various synthetic rubbers by first banding the natural rubber on a roll mill and then adding the cis-polybutadiene. In this example, several cis-polybutadiencs were utilized as prepared in the presence of benzone and a catalyst consisting of titanium tctraiodide (TH) and triisobutylaluminum (TIBA) under the following conditions.

Polymer- Percent Milli- Milli- Mol Mooney Polyization convermoles moles ratio ML-4 at mer Temp. sion TIBA TII TIBA/ 212 F.

F TTI 0 42 10.0 2.5 4/1 17 0 62 1a. 5 1. s 7. 5/1 is 0 64 9.0 1.8 5/1 is 0 25 7.5 1.5 5 1 16 0 58 10.0 2.0 5/1 120 0 87 11.0 2.2 5/1 37 Polymer X is a cis-polybutadiene blend of polymers A-D and has a blended Mooney (ML-4) of 14. The blend contained the following percents of the 4 polymers:

Percent. A 42.7

Polymer G is a polybutadiene prepared by emulsion polymerization Whereas polymer H is a /25 butadiene/ styrene polymer prepared by emulsion polymerization. Both of these polymers were prepared in processes employing a sulfoxylatc activated catalyst system at 41 F. The 75/25 butadienc/styrene was prepared by commercial process.

Goodrich flexoineter, 257 lbs/sq. in. load, 0.250 inch stroke, 200 F oven temperature. Reported as running time to failure of test specimen ll ASTM D1053-54'l (modified). Gehman torsional apparatus. Test specimens are 1.625 inches long, 0.125 inch wide and 0.077 inch thick. The angle of twist is measured at 5 0. intervals. Extrapolation to zero twist gives the freeze point.

12 Samples employed were strips 4 inches long and 0.5 inch wide. They were mounted in racks where they were elongated 25 percent and exposed to air containing 50 parts by volume 01' ozone per million parts of air. The samples were rated according to the following numerical sys em:

(1) Surface slightly dulled.

(2) First evidence of attack, bubbly" appearance on surface.

(3) Roughening of surface, no open cracks.

(4) First evidence of very minute cracks.

(5) Many minute shallow cracks.

(6) Longer shallow cracks.

(7) Deeper cracking, numerous cracks having appearances of very fine tight lace.

(8) More serious cracking, rowing quite deep.

(9) Lacework of deep cracks.

(l0) Cracks, both deep and numerous.

( Decreased in rate after second pass.

( Crystallizcd.

( Maximum run.

( Percent broken at 50.000 flexures.

(=) crystallized over a wide range.

(*) Estimated (interpolated value from stressstrain curves).

Polymers X, E, F, G and H were blended with natural rubber as above and compounded according to the following recipes:

Parts by Weight Polymer X, E Polymer G #1 8.8.

and F and H Polymer 100 50 100 50 100 No. 1 smoked sheet 5 50 High abrasion furnace black. 50 50 50 50 50 Zinc oxide 3 3 3 3 4 Stearic acid- 2 2 2 2 3 Antioxidant 1 1 1 1 1 Disproportionated rosin- 5 5 Sulfur 1. 75 1. 75 1. 75 1. 75 2 N cyolohexyl 2 benzothizt zolesulfenamide (accelerator) 9 Var. Var. Var Var 0. 4 Aromatic petroleum oil (plasticizer) Var. Var. 10 7. 5 5. 0

1 Same as in Example I. 9 Variable.

These compositions were cured for 30 minutes at 307 F. and the properties determined, employing the test procedures listed in the footnotes to Table I. The data obtained are given in Table II. Values in Table II listed With an (E) beside them are interpolated values while the dashes in this and other tables indicate that that particular value was not obtained.

When one compares the heat build up of the cis-polybutadiene blend with natural rubber with that of the blend of emulsion polymer with natural rubber, it is readily seen that cis-polybutadicne blend approximates the smoked sheet Whereas the emulsion blend approximates the emulsion polymer alone. This advantage for the cis-polybutadieno-natural rubber blend is material in fabrication of heavy duty tires.

TABLE II 50/50 blends of high and low Mooney eis-polybutadiene with natural rubber Polymer e X X E E F NSo l G G H H Parts by weight:

Butadiene polymer 100 50 100 50 100 100 50 100 50 Natural rubber 50 50 100 50 50 Plasticizer, phr, 2.5 10.0 7. 5.0 6.0 10.0 7 5 10.0 7. 5 Accelerator, phr. 1.0 0. 8 0.9 0.65 0.9 0. 4 1.6 1 0 1.2 0. 8

Processing Data Mooney, ML-4 at 212 F 14 120 37 100 44 52 compounded Mooney, MS 1% at 212 F 24. 5 31 67 68 35. 5 41. 5 34 34 30. 5 37 Extrusion at 250 F;

Inches/min 43. 3 38. 5 39. 0 32. 5 51. 4 32. 1 32. 5 44. 5 40. 0 Grams/min 83.0 82. 5 68.0 75. 0 104. 0 85. 0 76. 0 110. 0 88. 5 Ratin 11+ 11+ 3 8 11 9- 10 11+ 10+ Cured Minutes at 307 F,

Compression set, percen 18.0 17.0 11. 6 14. 8 17. 4 19. 9 20. 4 17. 2 20. 4 .3 300 percent modulus, p.s.i., 80 F 6 7 420 1, 150 1, 380 1, 300 1, 520 1, 350 1,400 1, 450 Tensile, p.s.i., 80 F 2, 250 a 2,800 2, 520 a 2,800 2,000 3, 050 2,120 3, 330 3, 680 a, 530 Elongation, percent, 80 F 365 9 420 e 5 5 380 475 370 540 600 540 200 F un tensile, n i 1, 135 1, 930 l, 325 1, 960 1, 120 2, 270 1, 125 I, 980 1, 660 2, 240 Heat build-up, A T, "F 56. 8 52. 7 38.8 49, 7 47. 9 47. 6 61. 5 58.1 63. 5 60.8 Resilience, percent 68. 9 67.0 76. 5 67.0 72. 0 67. 6 59.3 60.1 58.2 59. 0 Flex life, thousands of flexures to failure 4 0 26 0 0.7 29. 7 1.2 2. 7 24. 0 23.1 34. 8 Shore A hardness. 67 63 63 61 63 58. 5 60 60 62. 5 Tear Strength, lb/inch 7 8 550 185 455 495 255 450 420 520 Time to blowout, minutes 14. 0 20. 0 30. 0 9. 8 9. 0 8. 8 9. 8 9. 9 8. 6 Ozone rating, 3 days 3 2 2 3 1 2 3 3 4 Oven Aged 24 hours at 212 F,

Heat build-up, AT, "F 45. 9 47. 3 34. 4 46.6 39. 2 43. 9 55. 4 51. 7 54.1 55, 4 Resilience percent 75. 8 70. 3 79. 4 69. 9 77. 4 68.2 64. 8 65. 3 63. 4 62. 1 Shore A hardness. 71 66 64 67.5 63 62. 5 64. 5 64 65 1 Crumbled. 3 ASTMI 13624-54, Die A.

b Phr.=parts by weight per 100 parts rubber.

" Test discontinued-no blowout occurred.

Usually breaks to 15% by 50,000 flexures (was 11% at 10,000 fiexures). e Estimated.

Mooney viscosity 37 (ML-4 at 212 F.). G=Emulsion polybutadiene.

H=Butodiene-styrene copolymer.

Example 111 Still another series of runs were made in which natural rubber was blended with various synthetic rubbers.

Compounding recipes, parts by Weight These blends are made up as described 1n Example II. 45 Synthetic Natural The polymers, alone and in admixture wlth natural rub- 00 38 u her, were compounded according to the following recipes, an en S a one after which they were cured for 30 minutes at 307 F. A

romatic petroleum oil 5 5 i s l 11 12 b thl 1 11 a 2 -cyc o exy enzo azy su enami e ac- Compoundrng recipes r parts by Weight 00 eleretor) Variable 0. 4

Synthetic Natural 1 Same as in Example polymers rubber and blends alone After the polymers and blends were compounded and U0 0 f5 cured, the physical properties. of the materials were de- Polymer l 1 0 p High abrasionfumace mach 50 50 terrnlrled for both aged and unagcd stocks. The prop z ll-m 3 4 ertles of these stoclcs are tabulated below 1n Table III, 32 s and the test procedures used in determining the properties Disproportiona Variable 0 or 5 are described in the footnotes to Table I.

TABLE III.BLENDS 0F SYNTHETIC RUBBER WITH NATURAL RUBBER 1+ K-l- L+ No.1 No.1 Polymer B or blend J K L L J-l-L K+L- No.1 No. 1 N0. 1 8.8. 8.8.

Parts by weight:

Synthetic polymer 100 100 100 50 50 50 50 .50 0 0 Natural rubber or synthetic polymer 0 0 0 0 50 50 50 50 50 100 100 Accelerator, phr 0.8 1 6 1.2 1.2 1.0 1.4 0.6 1 1.0 0.8 0.4 0.4 Disproportionated rosin, phr 5 5 5 5 5 5. 5 5 5 Processing Data Mooney, ML-4 at 212 F 45 44 52 52 I G) 100 100 compounded Mooney, MS 1% at 212 F 50 32 33 34 38 32 42 29 30 28 39 Extrusion at 250 F.:

Inches/min 29.3 29.0 43.8 43.0 38.8 34.8 44.0 37.5 46.3 49.5 52.0 Grams/min 61.0 80.0 104.0 105.5 87.0 V 96.0 90.0 85.5 94.0 00.0 95.0 Rating... 5- 10+ 11+ 12- 7+ 10 10- 11- 11+ 11+ 11+ See footnotes at end of table.

1 Not measured. 2 Estimated.

I Polymer J is a cis-polybutadiene which was prepared by polymerizatlon at 41 F. using a 5/1 mol ratio of triisobutylaluminum/titanium tetraiodide. This polymer contained 95.5% cis linkage, 1.0% trans linkage and 3.5% vinyl linkage as determined by the complete infrared method described by Silas, Yates and Thornton in Determination of Unsaturation Distribution in Polybutadiene by Infrared Spectrometry,

Example IV TABLE IIICntinued Cured 30 Minutes At 307 F 1+ K+ L+ No. 1 No. 1 P ly s r blend J K L L J+L K+L No. 1 No. 1 No. 1 ss. 8,8.

5.8. v 8.8. S.S.

Compression set, percent..- 16. 0 24. 4 26. 2 19.1 21. 0 27. 4 18. 2 20. 2 22.7 24. 8 23. 9 300% Modulus, p.s.l., 80 1,060 1,170 1, 150 1, 740 1,070 1, 200 1,120 1,240 1, 210 1, 240 1,550 Tensile, p.s.i., 80 F- 2,150 3 2,500 3, 470 3, 750 2,510 2, 840 3,140 3, 380 3, 490 3, 460 3, 700 Elongation, percent, 470 2 480 665 550 525 535 585 610 635 605 535 F. max. tensile, 13.5.1 1, 430 1, 320 1, 590 1, 920 1, 020 l, 460 1, 920 2, 040 2, 350 2, 740 3, 020 Heat build-11p, A T, 41. 9 68. 3 75. 3 61. 58. 1 71.9 46. 9 58. 5 63. 5 47. 3 2. 6 Time to blowout, minutes 22. 3 7. 4 5. 0 11.3 15. 3 6. 3 14. 1 8. 8 6. 5 6.9 6.4 Flex life, thousands of fiexur 1.4 2. 8 3. 9 23.0 4. 5 3. 6 8. 0 20. 4 b 56.4 0 12. 5 s 15.0 Shore A hardness 62 57. 5 59 63. 5 59. 5 59 60 59 59. 5 58. 5 61. 5 Ozone rating, 8 day 7 10 10 9 10 8 8 9 3 2 Oven Aged 24 Hours At 212 F.

300% Modulus, p.s.i., 80 F 2,180 2,790 2,175 1,625 2,025 1,760 1, 675 1,950 Tensile, p.s.i., 80 F 3, 650 3,480 1, 650 3, 080 2, 280 3, 000 3,150 2, 680 2.810 Elongation, percent, 80 F" 470 370 255 400 380 410 490 450 410 Heat build-up, AT, 55. 8 53. 4 47.6 55.1 44. 9 51. 3 54. 1 40. 5 39. 2 Flex life, thousands of flextures to tail 2. 8 1. 5 0. 9 1. 1 s 47. 0 10. 7 8.4 33. 0 23.0 Shore A hardness 64. 5 68 65 65 63. 5 65 65. 5 62. 5 65 Analytical Chemistry 31, 529 (1959). Polymer K is identical to Polymer G (emulsion polybutadiene) of Example 11. Polymer L is identical to Polymer H (butadiene-styrene copolymer) 01' Example II. No. 1 5.8. is top grade smoked sheet natural rubber).

b Broke vertically instea of horizontally.

s Percent broken at 50,000 fiexures.

25 Still another series of runs were made in which natural Compounding recipes, rubber was blended with a butadiene polymer which con- Parts by tained a very high amount of cis-l,4-configuration. The C, 1 b my 0 V 1 b1 1 blends, and the runs in which the cis-polybutadienc and j i g gg ,121:" 100 g f g g 8 natural rubber were used alone, were compounded acu br furnance l 50 50 50 h f I b t Z1110 oxlde 3 3 3 cor mg to t e o ogvmg 1recipes. e (HS-l, -p0dy1u aigg 3 1 2 2 2 diene was re ared o merization in to uene iuent non ant 1 1 1 p p y P Disproportionatedr 5 Variable 5 using a. mol ratio of trnsobutylalummum to titanium Aromatic petroleum on 5 5 5 tetraiodide of 3.75/ 1.0. The cis-polybutadiene used in i g 3-55 Va 1. 52 the blends was a composite of polymers from 6 runs, all 35 r o of which were made at 20 F., using water as the short 1 Same asin Example L stop. The composite, which contained 2 parts/ 100 parts 2 Same m Example n rubber of phenyl-beta-naphthylamine, had a Mooney viscosity (ML-4) of 26, and a cis content of 91.4 per- After compounds, the rubbers were cured for 30 mmcent, a trans content of 4.4 percent, and a vinyl content 40 at EXWPt Where 'f The p y D pof 4.2 percent as determined by the infrared method of fifties 0f the (Red rubbers, which were detefmlnfid y Silas, Yates and Thornton referred :0 in th fo t-n t the test procedures referred to 1n the footnotes to Table I, to Table III. are recorded below in Table IV.

TABLE IV Polymer or blend M N P R S T V X Parts by weight:

Gis-polybutadiene 0 10 25 50 Natural rubber 100 90 75 50 25 10 0 50 Accelerator, phr 0. 0 0. 64 0- 7 0. 8 0. 9 0.96 1.00 0.8 Disproportionated-rosin, phr 5 5 5 5 5 5 5 0 Processing Data .Oompounded Mooney, MS 1% at 212 F 36. 5 36 34. 5 34 32.5 32. 5 41. 5

Extrusion at 195 F.: 2

ches/min--- 45. 5 48. 4 48. 5 47.9 46. 5 44. 0 41. 0 46. o Grams/mins5: 5 90. 5 94. 0 99. 0 101. 0 96. 5 91. 0 96. 5 Rating 11+ 11 11 11 11 11+ 11 11+ Cured 30 Minutes at 307 F.

' l Compression set, percent 18. 3 18.2 16. 6 l7. 8 18. 0 18. 5 19. 2 16. 5 300% Modulus, p.s.l., 80 1,300 1, 320 1,450 1, 320 1, 280 1, 230 1,100 1,500 Tensile strength, p.s.i., 80 F- 3, 050 3, 570 3, 480 3,080 2, 410 2,150 1 1, 900 3, 030 Elongation, percent, 80 11- l 575 555 560 550 460 440 l 440 500 200 F. max. tensile, p.s.i 2, 715 2,650 1 2, 500 1,935 1, 180 1, 030 915 1,780 Heat build-up, A T, F 46. 0 46.3 46.3 49.0 49. 3 4o. 3 51.0 49. 3 Resilience, percent- 68.6 68. 5 69. 1 68. 0 69. 4 71. 3 70. 9 71. 6 Time to blowout, minutes 9. 0 10. 5 14 4 15. 8 l 15 15. 5 16. 8 8. 8 Flex life, thousands of flexures to failure 2 15 2 16 i 14. 5 12.1 5. 6 2. 4 1. 3 18. 0 Shore A hardness 58 59 60. 5 60 62 62 62. 5 63 Tear strength at 80 F., lb.linch 455 530 420 430 280 V 530 See footnotes at end of table.

TABLE IVCntinued Oven Aged 24 Hours at 212 F.

Polymer or blend M N P R S '1 V X 300% modulus, p.s.i., 80 F 1 1, 575 1, 590 1,680 1-, 950 2,080 Tensile strength, p.s.i., 80 F.- 1 2, 200 2,100 2,010 1 2,050 1, 700 1, 620 1,550 2, 260 Elongation, percent, 80 F... l 415 375 360 1 340 265 240 220 310 Heat build-up, A '1, F 43.3 45. 6 43. 9 44. 9 45. 6 43. 6 48. 7 48. 7 Resilience, percent 71.1 08. 2 71. 9 72. 3 72. 8 76. 2 75. 5 72.9 Flex life, thousands of fiexures to failure. 35. 0 20. 4 47. 4 14. 6 1. 8 0. 1 0. 1 2 56. 7 Shore A hardness 61 62. 5 63.5 64. 5 67 68 66 1 Estimated. 2 Percent broken at 50,000 flexures.

Example V A series of runs was made wherein new 7:60 x tire The results of these tests are tabulated below.

carcasses were retreaded by using half and half retread PM Tqtal Miles] Rating Tread construction, i.e., /2 of the tire circumference 1s covered Tlre Tread Sulfur mnes NR=100 crackmg with one tread composition and V; with a second tread composmon. These ores were placed on a Dodge sta- Lu 50/50 testbkmd 2 97782 968 125 None 111011 Wagon operatmg on a regular route in the southwest. Natural rubber 2 9,782 77. 6 100 Extensive surface. The C18 polybutadiene Tabb?- was one P m 50/50 test blend-. 1.25 7,341 04.7 124 None. Example I, employmg a trnsobutylalurmnum-utamum Naturalrubber.. 2 7, 511 76.2 100 Extensive S111 306. tetra1od1de catalyst. Infrared exarnlnatron of polymer 50/50 testblend" 2 127245 ml 121 None used 1n the tests according to the method of Silas, Yates Naturalrubber-- 2 12, 245 70.4 100 Extensive S111 ace. and Thornton referred to 1n the footnote to Table III in 50l50testb1end 2 3,236 73.9 116 mm d1cated that the polymer contamed 95 percent ms, 2 per- 0 SBR 1.75 3, 235 53.5 100 Do. cent trans, and 3 percent vinyl configuration. These g gfffff ff 75 328 13 g3 g3: polymers contanied 1.8 percent phenyl-fi naphthylamine 50/50 test blend.. p 2, g g Bo! I 0. ant oxidant. All polymers were gel free. Mooney v s- 50/50 testblend 6,553 753 118 cos1t1es were to 45 (ML-4 at 212 F.) unless otherw1se 3 1.75 8, 13-3 1( go.

5 es en o. lndlcated- 35 1.75 9, 722 71.8 100 Do.

The SBR (styrene-butadlene copolymer rubber) was a gags test blend. 1 2 337 p4 go. copolymer containing 24 percent bound styrene prepared 7 00 by emulsion polymerization in an iron-activated recipe at 1 1. h h h 41 F. ThlS rubber had a mean ML-4 Mooney at 212 a $533,133.5 51,; $8,553, 35 lazylsuuenamlde F. of 52.

The natural rubber was premasticated #1 Smoked Sheets. The rubbers were compounded according to the follow- Example VI mg compounding recipes. 45

Four truck tires were capped /2 with blend as in EX- Compounding recipes, ample V and with natural rubber. The blend had a D51Its by Weight 2 parts by weight of sulfur per 100 parts of rubber in the compounding recipe whereas with the natural rubber 2.25 55 3 figg gi SBR parts by weight of sulfur per 100 parts of rubber was used. The cis-po1ybutadiene used in preparing the test blend was a blend of polymers obtained from 6 runs carried out C1s-p0lybutad1ene 50 Natural rubber 50 100 accordlng to the Sm1th et al. method as descnbed 1n Exgag-55, furnace blacknrnnfinjj 50 50 28 ample I. The products from two of these runs were ex- Zinc oxide 3 3 3 amined by infrared analysis according to the method of Steam: ac1d 3 3 1 Antioxidant 1 1 1 Sllas, Yates and Thornton menuoned 1n the footnote D's o 0 to at d s- 5 f f g 2 5 3 on 5 5 10 to Example III and found to contam 93.6 and 94.4 ms Sulfur. Variable 2 1.75 1,4-add1t1on. These tires were 10.00 x 20 truck tires and ggfiffi fff'ffi fffi ffffi ff fl 6 Were placed on trucks to be tested by the Armstrong Test N-cyclohexyl-2-benzothiazylsulfenamide 0. 4 .2 Fleet at San Antonio, Texas. At the end of approximately 3,000 miles, the following data was obtained.

Total Miles/ Rating Tread Tire Tread miles .001 NR= cracking Remarks 1 Natural rubber.... 3,128 41 100 None.... 50/50 test blend.-.- 3,128 45 ...do.. 2 Natural rubber..- 3,128 40 50/50 test blend.... 3 128 44 3 Natural rubber. 3,128 40 50/50 test blend.-.- 3,128 43 1 a 4 Natural rubber.. 2,919 53 }Oarcass failure between shoulder and 50/50 test blend... 2, 919 65 side wall.

From the above examples, it can be seen that tires treaded with the blend of cis-polybutadienc and natural rubber are generally superior to the natural rubber alone and to a commercially acceptable cold rubber.

Example VII Butadiene was polymerized to cis-polybutadiene in a series of runs using a triisobutylaluminum-titanium tetra iodide catalyst system.' The polymers were blended to give a product having the following characteristics:

Infrared examination of the cis-polybutadiene by the method of Silas, Yates and Thornton mentioned in the footnote to Example III indicated that the polymer contained 94 percent cis 1,4-addition, 2.3 percent trans 1,4- addition and 3.7 percent 1,2-addition. The polymer contained 1.79 weight percent phenylbeta-naphthylamine.

The cis-polybutadiene was blended in equal weight proportions with a synthetic cis-polyisoprene and with natural rubber. The cis-polyisoprene was a sample of Coral rubber (Firestone Tire and Rubber Co.) Which is described more completely in Coral Rubber-A Cis 1,4-Po1yisoprene, Ind. and Eng. Chem. 48, 778 (1956). Based on a comparison with a standard sample of natural rubber containing 98 percent cis 1,4-addition, this cis-polyisoprene was determined by infrared examination to contain 89i2 percent cis 1,4-addition and 7.61:0.2 percent 3,4-addition. These blends were compounded using the following formulations:

High abrasion furnace black.

=Physical mixture containing 65 percent by weight of a complex diarylamineketone reaction product and 35 percent by weight of N,N-diphenyl-p-phenylenediamine.

3 A highly aromatic oil.

4 A disproportionated pale rosin stable to heat and light.

5 N-oxydiethylene-Z-benzothiazylsulfenamide.

One set of samples was cured for 30 minutes while another set was cured for 45 minutes, both at 292 F. Results ofdeterminations of physical properties are set forth below in Table V. The physical properties were determined according to the test procedures referred to in the footnotes to Table I.

TABLE V 30 minute cure at 292F.:

Compression set, percent 27. 5 34. 8 300% modulus, p.s.i., 80F 1,100 980 Tensile, p.s.i., 80F 3, 480 3, 310 Elongation, percent, 80F 570 625 Heat build-up, A T, F V 43. 3 53.0

" Resilience, percent .L 70. 7 67. 9 Shore A hardness 56. 5 54. 5

45 Minute Cure at 292F.:

300% modulus, p.s.i., 80F 1, 040 1,190 Tensile, p.s.i., 80F 3, 080 3, 470 Elongation, percent, 80F. 510 590 Heat build-up, A T, F 40. 9 45. 9 Resilience, percent 71. 7 70. 6

- 50/50 blend of cis-polybutadiene and cis-polyisoprene. 1 50/50 blend of cis-polybutadiene and natural rubber.

16 The foregoing data show that the all-synthetic composition (cis-polybutadiene and cis-polyisoprene) has properties which compare favorably with those of the natural rubber composition.

Example VIII A cis-polybutadiene prepared as described in Example I was blended in equal weight proportions with a synthetic cis-polyisoprene, which was a sample of Natsyn rubber (Goodyear Tire and Rubber Co.) (Chem. & Eng. News, Ian. 19, 1959, p. 50). Based on a standard sample of natural rubber containing 98 percent cis 1,4-additi0n, this cis-polyisoprene was determined by infrared examination to contain :2 percent cis l,4-addition and 4.5 $0.2 percent 3,4-addition. This blend and also synthetic cis-polyisoprene alone and natural rubber alone were compounded in accordance with the following recipes:

TABLE VI MS 1% at 212 F 26 18 29 Scorch at 280 F., min. to .5 point Mooney rise. 26 18. 5 0. 5 Extrusion rating, Garvey die 11+ 11 11+ v 10 moles/cc. l. '74 1. 53 1. 88 Compression set, percent. 19. 8 '21. 9 15. 4 300% Modulus, p.s.i., 80 1,565 1,640 2, Tensile, p.s .i., 80 F 2,680 3,035 3, 790 Elongation, percent, 80 F 440 495 485 Heat build-up, AT, 40. 5 43.9 39.5 Resilience, percent 72. 3 71. 9 73. 2 Time to blowout, minutes- 22.0 10. 8 l4. 8 AT, 9 F. at 10 min 90. 0 175. 4 107. 6 Shore A hardness.-- 65 63. 5 70 Abrasion loss, grams 7.07 10. 60 8. 42

1 50/50 blend of cis-polybutacliene and cis-polyisoprene.

I Cis-polyisoprene.

3 Natural rubber.

4 Determined by the swelling method of Kraus as given in Rubber World, October 1946. This value is the number of efiective network chains per unit volume of rubber. The higher the number, the more the rubber is crosslinked (vulcanized).

i Determined by noting the loss in weight of a doughnut shaped rubber wheel which has been subjected to the abrasive action of a carborundum wheel on the angle abrader for a certain length of time. The wheel used is 24 inches in diameter, 1% inches thick, Grade M, Vitreous, grain size No. 36 alundum purchasedirom Norton Com any, Worcester, Mass. The normal test conditions are 15 angle, 33 pounds load and 3,000 revolutions.

' time, and abrasion loss.

1 7 Example IX The rubber compounds 1, 2 and 3 of Example VIII were used to make three-way retreads on 7.60 X 15 tire carcasses. The following results were obtained after the tires were run 3,083 miles:

1 Natural rubbeFIOO.

As will be evident to those skilled in the art, many variations and modifications can be produced which fall within the spirit and scope of the disclosure of this invention.

We claim:

1. As a new composition of matter, a blend of rubbers comprising (1) in the range of 1 to 90 parts by weig t of a polybutadiene formed by cis 1,4-, trans 1,4- and 1, addition of 1,3-butadiene, at least 75 percent of said polybutadiene being formed by cis 1,4-additi'on of 1,3- butadiene and (2) in the range of 90 to 10 parts by weight of a cis-polyisoprene, the aforementioned parts by weight ranges being based on 100 parts by weight of total rubbers contained in the blend.

2. The vulcanized product of claim 1.

3. The composition of claim '1 in which said cis-polyisoprene is natural rubber.

4. The composition of claim 1 in which said cis-polyisoprene is a synthetic cis-polyisoprene.

5. As a new composition of matter, a blend of rubbers comprising (1) in the range of 40 to 50 parts by weight of a polybutadiene formed by cis 1,4-, trans 1,4- and 1,2- "addition of 1,3 butadiene, at least 85 percent of said polybutadiene being formed by cis 1,4-addition of 1,3-butadiene and (2) in the range of 60 to 50 parts by weight of a cis-polyisoprene, the aforementioned parts by weight ranges being based on 100 parts by weight of total rubbers contained in the blend.

6. The composition of claim 5 in which said polybutadiene has a Mooney ML-4 viscosity in the range or 20 to 60 as measured on a Mooney viscosimeter at 212 F.

7. A method of preparing a blend of rubbers which comprises blending in the range of to 90 parts by weight of a polybutadiene formed by cis 1,4-, trans '1,4-

18 and 1,2-addition, at least percent of said polybutadiene being formed by cis 1,4-addition of 1,3-butadiene within the range of to 10 parts by weight of a cis-p'olyisoprene, the aforementioned parts by weight ranges being based on parts by weight of total rubbers contained in the blend; and vulc'anizing the resulting blend.

8. A method of preparing a blend of rubbers which comprises blending in the range of 25 to 75 parts by weight of a polybutadiene formed by cis 1,4-, trans 1,4- and 1,2-addition, at least 75 percent of said polybutadiene being formed by cis 1,4-addition of 1,3-butadiene within the range of 75 to 25 parts by weight of a cis-polyisoprene, the aforementioned parts by weight ranges being based on 100 parts by weight of total rubbers contained in the blend; and said polybutadiene having a Mooney ML-4 viscosity in the range of 10 to as measured on a Mooney viscosimeter at 212 and incorporating a vulcanizing agent into the resulting blend.

9. A method of preparing a blend of rubbers which comprises blending in the range of 40 to 50 parts by weight of a polybutadiene formed by cis 1.4-, trans 1,4- and 1,2-addition, at least 75 percent of said polybutadiene being formed by cis 1,4-addition of 1,3-b-utadiene within the range of 60 to 50 parts by weight of a cis-polyisoprene, the aforementioned parts by weight ranges being based on 1 00 parts by weight of total rubbers contained in the blend and said polybutadiene having a Mooney ML-4 viscosity in the range of 20 to 60 as measured on a Mooney viscosimeter at 212 F.; incorporating sulfur into the resulting blend; and heating the blend so as to efiect vulcanization.

10. In an automotive tire comprising 'a carcass and tread, the improvement which comprises a tread prepared from the composition of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,688,605 Tucker Sept. 7, 1954 2,832,759 Nowlin et a1. Apr. 29, 1958 2,953,556 Wolfe et a1. Sept. 20, 1960 2,977,349 Brockway et a1 Mar. 28, 1961 OTHER REFERENCES Binder: Microstructures of Polybutadiene and Butadiene-Styrene Copolymers, Ind. Eng. Chem, volume 46, No. 8, August 1954, pages 1727-1730.

Rubber World, volume 138, No. 2, May 1958, page 280 relied upon. 

1. AS A NEW COMPOSITION OF MATTER A BLEND OF RUBBERS COMPRISING (1) IN THE RANGE OF 10 TO 90 PARTS BY WEIGHT OF A POLYBUTADIENE FORMED BY CIS 1,4-, TRANS 1,4- AND 1,2ADDITION OF 1,3-BUTADIENE, AT LEAST 75 PERCENT OF SAID POLYBUTADIENE BEING FORMED BY CIS 1,4-ADDITION OF 1,3BUTADIENE AND (2) IN THE RANGE OF 90 TO 10 PARTS BY WEIGHT OF A CIS-POLYISOPRENE, THE AFOREMENTIONED PARTS BY WEIGHT RANGES BEING BASED ON 100 PARTS BY WEIGHT OF TOTAL RUBBERS CONTAINED IN THE BLEND.
 10. IN AN AUTOMOTIVE TIRE COMPRISING A CARCASS AND TREAD, THE IMPROVEMENT WHICH COMPRISES A TREAD PREPARED FROM THE COMPOSITION OF CLAIM
 1. 